Direct frictional contact between relatively moving surfaces even in the presence of a lubricant can cause wear of the surfaces. The elimination of wear is an ideal goal which is approached by blending the lubricating media with additives which can reduce the wear. The most suitable antiwear additives are those that help to create and maintain a persistent film of lubricant even under severe conditions which would tend to dissipate the lubricant film such as high temperatures which thin the lubricant film and extreme pressures which squeeze the lubricant film away from the contacting surfaces. Wear is most serious in internal combustion engines, diesel engines and gasoline engines in which metal parts are exposed to sliding, rolling and other types of forceful, frictional mechanical contact. Specific areas of wear occur in the gears, particularly hypoid gears which are under high loads, piston rings and cylinders and bearings such as ball, sleeve and roller bearings. Since antiwear lubricants are made by incorporating antiwear additives into the lubricating fluid, compatibility of the additive is important. Compatibility is a problem encountered in the art because the antiwear functionality is usually polar which makes that portion insoluble in the lubricant. It is desirable to make antiwear additives which maintain the antiwear functionality while at the same time are soluble in the lubricant fluid.
Rust prevention is important in machines which are made from ferrous alloys, other than stainless steel, which are subject to rusting upon exposure to humid air. Mineral oils notoriously do not have good rust preventative properties and have; therefore, been mixed with appropriate antirust additives. While synthetic oils have better antirust properties they too can benefit from compatible antirust additives. Antirust additives are usually hydrophobic polar compounds which are adsorbed at the metal surface to shield the surface from exposure to corrosive compounds present in the environment. Known antirust additives of this kind include esters of phosphorus acids. Other antirust additives have the ability to neutralize the acidity of the lubricant as oxidation occurs. Antirust additives of this kind which are particularly useful under relatively high temperature conditions are nitrogenous compounds; e.g. alkyl amines and amides.
Oxidation of a lubricating oil occurs during ordinary, as well as severe, conditions and use. The properties of the oil change due to contamination of the oil and chemical changes in the oil molecules. Oxidation can lead to bearing corrosion, ring sticking, lacquer and sludge formation and excessive viscosity. Acid and peroxide oxidation products can promote corrosion of metal parts, particularly in bearings. The presence of an antioxidant can have a profound effect upon the rate of oxidation of the lubricating oil. Known antioxidants include hydroxy compounds, such as phenols, nitrogen compounds such as amines and phosphorothioates, particularly zinc dithiophosphates.
The use of phosphorodithioate compositions, specifically the zinc dithiophosphates have been known as multifunctional antiwear, peroxide decomposing and bearing corrosion inhibiting additives.
Urea and urethane derivatives have also been described as having good antioxidant characteristics and antiwear properties in lubricants. For example U.S. Pat. No. 3,980,574 describes lubricants containing diorganophosphorus derivatives of urethane as antiwear agents. The additive described in this patent does not contain a phosphorodithioate functional group.
U.S. Pat. No. 4,235,730 describes a polyurethane derived from a diisocyanate and a diol having dispersant properties for lubricants and fuels.
U.S. Pat. No. 4,897,087 describes a reaction product of a polyether and a polyamine which are linked together by a diisocyanate having ashless dispersant and detergent properties for fuels.